Method for measuring the magnetotelluric response to the earth&#39;s subsurface

ABSTRACT

A method for measuring magnetotelluric response of the Earth includes measuring transient controlled source electromagnetic response of the subsurface below a body of water over a plurality of actuations of an electromagnetic transmitter. The transient response measurements are stacked. The stacked transient responses are subtracted from measurements of total electromagnetic Earth response over a time period including the plurality of transient response measurements to generate the magnetotelluric response.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to the field of marine electromagneticgeophysical surveying. More specifically, the invention relates tocables and related apparatus for acquiring, recording and transmittingelectromagnetic signals produced for subsurface Earth surveying.

2. Background Art

Electromagnetic geophysical surveying includes “controlled source” andnatural source electromagnetic surveying. Controlled sourceelectromagnetic surveying includes imparting an electric field or amagnetic field into the Earth formations, those formations being belowthe sea floor in marine surveys, and measuring electric field amplitudeand/or amplitude of magnetic fields by measuring voltage differencesinduced in electrodes, antennas and/or interrogating magnetometersdisposed at the Earth's surface, or on or above the sea floor. Theelectric and/or magnetic fields are induced in response to the electricfield and/or magnetic field imparted into the Earth's subsurface, andinferences about the spatial distribution of conductivity of the Earth'ssubsurface are made from recordings of the induced electric and/ormagnetic fields.

Natural source electromagnetics includes multi-components ocean bottomreceiver stations and by taking the ratio of perpendicular fieldcomponents, one can eliminate the need to know the natural source.Hereto, natural source electromagnetics for marine applications has beenrestricted to autonomous recording stations.

Controlled source electromagnetic surveying known in the art includesimparting alternating electric current into formations below the seafloor. The alternating current has one or more selected frequencies.Such surveying is known as frequency domain controlled sourceelectromagnetic (f-CSEM) surveying. f-CSEM surveying techniques aredescribed, for example, in Sinha, M. C. Patel, P. D., Unsworth, M. J.,Owen, T. R. E., and MacCormack, M. G. R., 1990, An active sourceelectromagnetic sounding system for marine use, Marine GeophysicalResearch, 12, 29-68. Other publications which describe the physics ofand the interpretation of electromagnetic subsurface surveying include:Edwards, R. N., Law, L. K., Wolfgram, P. A., Nobes, D. C., Bone, M. N.,Trigg, D. F., and DeLaurier, J. M., 1985, First results of the MOSESexperiment: Sea sediment conductivity and thickness determination, ButeInlet, British Columbia, by magnetometric offshore electrical sounding:Geophysics 50, No. 1, 153-160; Edwards, R. N., 1997, On the resourceevaluation of marine gas hydrate deposits using the sea-floor transientelectric dipole-dipole method: Geophysics, 62, No. 1, 63-74; Chave, A.D., Constable, S. C. and Edwards, R. N., 1991, Electrical explorationmethods for the seafloor: Investigation in geophysics No 3,Electromagnetic methods in applied geophysics, vol. 2, application, partB, 931-966; and Cheesman, S. J., Edwards, R. N., and Chave, A. D., 1987,On the theory of sea-floor conductivity mapping using transientelectromagnetic systems: Geophysics, 52, No. 2, 204-217.

Other publications of interest in the technical field of electromagneticsurveying include Edwards, N., 2005, Marine controlled sourceelectromagnetics: Principles, Methodologies, Future commercialapplications: Surveys in Geophysics, No. 26, 675-700; Constable, S.,2006, Marine electromagnetic methods—A new tool for offshoreexploration: The Leading Edge v. 25, No. 4, p. 438-444; Christensen, N.B. and Dodds, K., 2007, 1D inversion and resolution analysis of marineCSEM data, Geophysics 72, WA27; Chen, J., Hoversten, G. M., Vasco, D.,Rubin, Y., and Hou, Z., 2007, A Bayesian model for gas saturationestimation using marine seismic AVA and CSEM data, Geophysics 72, WA85;Constable, S. and Srnka, L. J., 2007, An introduction to marinecontrolled-source electromagnetic methods for hydrocarbon exploration,Geophysics 72, WA3; Evans, R. L., 2007, Using CSEM techniques to map theshallow section of seafloor: From the coastline to the edges of thecontinental slope, Geophysics 72, WA105; Damet, M., Choo, M. C. K.,Plessix, R. D., Rosenquist, M. L., Yip-Cheong, K., Sims, E., and Voon,J. W. K., 2007, Detecting hydrocarbon reservoirs from CSEM data incomplex settings: Application to deepwater Sabah, Malaysia, Geophysics72, WA97; Gribenko, A. and Zhdanov, M., 2007, Rigorous 3D inversion ofmarine CSEM data based on the integral equation method, Geophysics 72,WA73; Li, Y. and Key, K. 2007, 2D marine controlled-sourceelectromagnetic modeling: Part 1—An adaptive finite-element algorithm,Geophysics 72, WA51; Li, Y. and Constable, S., 2007, 2D marinecontrolled-source electromagnetic modeling: Part 2—The effect ofbathymetry, Geophysics 72, WA63; Scholl, C. and Edwards, R. N., 2007,Marine downhole to seafloor dipole-dipole electromagnetic methods andthe resolution of resistive targets, Geophysics 72, WA39; Tompkins, M.J. and Srnka, L. J., 2007, Marine controlled-source electromagneticmethods—Introduction, Geophysics 72, WA1; Um, E. S. and Alumbaugh, D.L., 2007, On the physics of the marine controlled-source electromagneticmethod, Geophysics 72, WA13; Dell'Aversana, P., 2007, Improvinginterpretation of CSEM in shallow water, The Leading Edge 26, 332;Hokstad, K., and Rosten, T., 2007, On the relationships between depthmigration of controlled-source electromagnetic and seismic data, TheLeading Edge 26, 342; Johansen, S. E., Wicklund, T. A. and Amundssen, H.E. F., 2007, Interpretation example of marine CSEM data, The LeadingEdge 26, 348; and MacGregor, L., Barker, N., Overton, A., Moody, S., andBodecott, D., 2007, Derisking exploration prospects using integratedseismic and electromagnetic data—a Falkland Islands case study, TheLeading Edge 26, 356.

Following are described several patent publications which describevarious aspects of electromagnetic subsurface Earth surveying. U.S. Pat.No. 5,770,945 issued to Constable describes a magnetotelluric (MT)system for sea floor petroleum exploration. The disclosed systemincludes a first waterproof pressure case containing a processor,AC-coupled magnetic field post-amplifiers and electric field amplifiers,a second waterproof pressure case containing an acousticnavigation/release system, four silver-silver chloride electrodesmounted on booms and at least two magnetic induction coil sensors. Theseelements are mounted together on a plastic and aluminum frame along withflotation devices and an anchor for deployment to the sea floor. Theacoustic navigation/release system serves to locate the measurementsystem by responding to acoustic “pings” generated by a ship-board unit,and receives a release command which initiates detachment from theanchor so that the buoyant package floats to the surface for recovery.The electrodes used to detect the electric field are configured asgrounded dipole antennas. Booms by which the electrodes are mounted ontoa frame are positioned in an X-shaped configuration to create twoorthogonal dipoles. The two orthogonal dipoles are used to measure thecomplete vector electric field. The magnetic field sensors aremulti-turn, Mu-metal core wire coils which detect magnetic fields withinthe frequency range typically used for land-based MT surveys. Themagnetic field coils are encased in waterproof pressure cases and areconnected to the logger package by high pressure waterproof cables. Thelogger unit includes amplifiers for amplifying the signals received fromthe various sensors, which signals are then provided to the processorwhich controls timing, logging, storing and power switching operations.Temporary and mass storage is provided within and/or peripherally to theprocessor. There is no active source in such MT methods, which rely uponnaturally occurring EM fields.

U.S. Pat. No. 6,603,313 B1 issued to Srnka discloses a method forsurface estimation of reservoir properties, in which average earthresistivities above, below, and horizontally adjacent to specificallylocated subsurface geologic formations are first determined or estimatedusing geological and geophysical data in the vicinity of the subsurfacegeologic formation. Then dimensions and probing frequency for anelectromagnetic source are determined to substantially maximizetransmitted vertical and horizontal electric currents at the subsurfacegeologic formation, using the location and the average earthresistivities. Next, the electromagnetic source is activated at or nearthe sea floor, approximately centered above the subsurface geologicformation and a plurality of components of electromagnetic response ismeasured with a receiver array. Geometrical and electrical parameterconstraints are determined, using the geological and geophysical data.Finally, the electromagnetic response is processed using the geometricaland electrical parameter constraints to produce inverted vertical andhorizontal resistivity depth images. Optionally, the invertedresistivity depth images may be combined with the geological andgeophysical data to estimate the reservoir fluid and shaliness(fractional volume in the formation of clay-bearing rocks called“shale”) properties.

U.S. Pat. No. 6,628,110 B1 issued to Eidesmo et al. discloses a methodfor determining the nature of a subterranean reservoir whose approximategeometry and location are known. The disclosed method includes: applyinga time varying electromagnetic field to the strata containing thereservoir; detecting the electromagnetic wave field response; andanalyzing the effects on the characteristics of the detected field thathave been caused by the reservoir, thereby determining the content ofthe reservoir, based on the analysis.

U.S. Pat. No. 6,541,975 B2 issued to Strack discloses a system forgenerating an image of an Earth formation surrounding a boreholepenetrating the formation. Resistivity of the formation is measuredusing a DC measurement, and conductivity and resistivity of theformations are measured with a time domain signal or AC measurement.Acoustic velocity of the formation is also measured. The DC resistivitymeasurement, the conductivity measurement made with a time domainelectromagnetic signal, the resistivity measurement made with a timedomain electromagnetic signal and the acoustic velocity measurements arecombined to generate the image of the Earth formation.

International Patent Application Publication No. WO 0157555 A1 disclosesa system for detecting a subterranean reservoir or determining thenature of a subterranean reservoir whose position and geometry is knownfrom previous seismic surveys. An electromagnetic field is applied by atransmitter on the seabed and is detected by antennae also on theseabed. A refracted wave component is sought in the wave field response,to determine the nature of any reservoir present.

International Patent Application Publication No. WO 03048812 A1discloses an electromagnetic survey method for surveying an areapreviously identified as potentially containing a subsea hydrocarbonreservoir. The method includes obtaining first and second survey datasets with an electromagnetic source aligned end-on and broadsiderelative to the same or different receivers. The invention also relatesto planning a survey using this method, and to analysis of survey datataken in combination so as to allow the galvanic contribution to thesignals collected at the receiver to be contrasted with the inductiveeffects, and the effects of signal attenuation (which are highlydependent on local properties of the rock formation, overlying water,and air at the survey area). This is very important to the success ofusing electromagnetic surveying for identifying hydrocarbon reserves anddistinguishing them from other classes of subsurface formations.

U.S. Pat. No. 6,842,006 B1 issued to Conti et al. discloses a sea-floorelectromagnetic measurement device for obtaining underwatermagnetotelluric (MT) measurements of earth formations. The deviceincludes a central structure with arms pivotally attached thereto. Thepivoting arms enable easy deployment and storage of the device.Electrodes and magnetometers are attached to each arm for measuringelectric and magnetic fields respectively, the magnetometers beingdistant from the central structure such that magnetic fields presenttherein are not sensed. A method for undertaking sea floor measurementsincludes measuring electric fields at a distance from the structure andmeasuring magnetic fields at the same location.

U.S. patent application Publication No. 2004/232917 and U.S. Pat. No.6,914,433 Detection of subsurface resistivity contrasts with applicationto location of fluids (Wright, et al) relate to a method of mappingsubsurface resistivity contrasts by making multichannel transientelectromagnetic (MTEM) measurements on or near the Earth's surface usingat least one source, receiving means for measuring the system responseand at least one receiver for measuring the resultant earth response.All signals from each source-receiver pair are processed to recover thecorresponding electromagnetic impulse response of the earth and suchimpulse responses, or any transformation of such impulse responses, aredisplayed to create a subsurface representation of resistivitycontrasts. The system and method enable subsurface fluid deposits to belocated and identified and the movement of such fluids to be monitored.

U.S. Pat. No. 5,467,018 issued to Rueter et al. discloses a bedrockexploration system. The system includes transients generated as suddenchanges in a transmission stream, which are transmitted into the Earth'ssubsurface by a transmitter. The induced electric currents thus producedare measured by several receiver units. The measured values from thereceiver units are passed to a central unit. The measured valuesobtained from the receiver units are digitized and stored at themeasurement points, and the central unit is linked with the measurementpoints by a telemetry link. By means of the telemetry link, data fromthe data stores in the receiver units can be successively passed on tothe central unit.

U.S. Pat. No. 5,563,913 issued to Tasci et al. discloses a method andapparatus used in providing resistivity measurement data of asedimentary subsurface. The data are used for developing and mapping anenhanced anomalous resistivity pattern. The enhanced subsurfaceresistivity pattern is associated with and an aid for finding oil and/orgas traps at various depths down to a basement of the sedimentarysubsurface. The apparatus is disposed on a ground surface and includesan electric generator connected to a transmitter with a length of wirewith grounded electrodes. When large amplitude, long period, squarewaves of current are sent from a transmission site through thetransmitter and wire, secondary eddy currents are induced in thesubsurface. The eddy currents induce magnetic field changes in thesubsurface which can be measured at the surface of the earth with amagnetometer or induction coil. The magnetic field changes are receivedand recorded as time varying voltages at each sounding site. Informationon resistivity variations of the subsurface formations is deduced fromthe amplitude and shape of the measured magnetic field signals plottedas a function of time after applying appropriate mathematical equations.The sounding sites are arranged in a plot-like manner to ensure thatareal contour maps and cross sections of the resistivity variations ofthe subsurface formations can be prepared.

Other U.S. Patent documents that provide background informationconcerning the present invention include the following:

U.S. Pat. No. 4,535,292 Transmitter for an electromagnetic survey systemwith improved power supply switching system (Ensing).

U.S. Pat. No. 5,130,655 Multiple-coil magnetic field sensor withseries-connected main coils and parallel-connected feedback coils(Conti).

U.S. Pat. No. 5,877,995 Geophysical prospecting (Thompson et al.).

U.S. Pat. No. 5,955,884 Method and apparatus for measuring transientelectromagnetic and electrical energy components propagated in an earthformation (Payton et al.).

U.S. Pat. No. 6,188,221 Method and apparatus for transmittingelectromagnetic waves and analyzing returns to locate underground fluiddeposits (Van de Kop et al.).

U.S. Pat. No. 6,225,806 Electroseismic technique for measuring theproperties of rocks surrounding a borehole (Millar et al.).

U.S. Pat. No. 6,339,333 Dynamic electromagnetic methods for directprospecting for oil (Kuo).

U.S. Pat. No. 6,628,119 Method and apparatus for determining the contentof subterranean reservoirs (Eidesmo, et al).

U.S. Pat. No. 6,664,788 Nonlinear electroseismic exploration (Scott C.Hombostel, et al).

U.S. Pat. No. 6,696,839 Electromagnetic methods and apparatus fordetermining the content of subterranean reservoirs (Svein Ellingsrud etal).

U.S. Pat. No. 6,717,411 Electromagnetic method and apparatus fordetermining the nature of subterranean reservoirs using refractedelectromagnetic waves (Ellingsrud, et al).

U.S. Pat. No. 6,859,038 Method and apparatus for determining the natureof subterranean reservoirs using refracted electromagnetic waves (SveinEllingsrud, et al).

U.S. Pat. No. 6,864,684 Electromagnetic methods and apparatus fordetermining the content of subterranean reservoirs (Ellingsrud, et al).

U.S. Pat. No. 6,864,684 Electromagnetic methods and apparatus fordetermining the content of subterranean reservoirs (Ellingsrud, et al).

U.S. Pat. No. 7,023,213 Subsurface conductivity imaging systems andmethods (Edward Nichols).

U.S. Pat. No. 7,038,456 Method and apparatus for determining the natureof subterranean reservoirs (Ellingsrud, et al).

U.S. Pat. No. 7,042,801 System for geophysical prospecting using induceelectrokinetic effect (Andrey Berg).

U.S. Pat. No. 7,126,338 Electromagnetic surveying for hydrocarbonreservoirs (MacGregor, Lucy et al.).

U.S. Pat. No. 7,141,968 Integrated sensor system for measuring electricand/or magnetic field vector components (Hibbs, et al).

U.S. Pat. No. 7,141,987 Sensor system for measurement of one or morevector components of an electric field (Hibbs, et al).

U.S. Pat. No. 7,145,341 Method and apparatus for recovering hydrocarbonsfrom subterranean reservoirs (Ellingsrud, et al).

U.S. Pat. No. 7,191,063 Electromagnetic surveying for hydrocarbonreservoirs (Tompkins).

U.S. Pat. Appl. Pub. No. 2006/0091889 Method and apparatus fordetermining the nature of subterranean reservoirs (Ellingsrud, Svein etal) application Ser. No. 11/301,010 filed on Dec. 12, 2005., granted asU.S. Pat. No. 7,202,669 on Apr. 10, 2007.

U.S. Pat. Appl. Pub. No. 2006/0129322 Electromagnetic surveying forhydrocarbon reservoirs (MacGregor, Lucy et al)

U.S. Pat. Appl. Pub. No. 2006/0132137 Electromagnetic surveying forhydrocarbon reservoirs (MacGregor, Lucy et al).

U.S. Pat. Appl. Pub. No. 2006/0197532 Method and apparatus fordetermining the nature of submarine reservoirs (Eidesmo, Terje et al).

U.S. Pat. Appl. Pub. No. 2007/0021916 Electromagnetic surveying forhydrocarbon reservoirs (MacGregor, Lucy et al).

41. U.S. Pat. Appl. Pub. No. 2007/0075708, ELECTROMAGNETIC SURVEY SYSTEMWITH MULTIPLE SOURCES (Reddig, Ransom et al).

A typical f-CSEM marine survey can be described as follows. A recordingvessel includes cables which connect to electrodes disposed near the seafloor. An electric power source on the vessel charges the electrodessuch that a selected magnitude of alternating current, of selectedfrequency or frequencies, flows through the sea floor and into the Earthformations below the sea floor. At a selected distance (“offset”) fromthe source electrodes, receiver electrodes are disposed on the sea floorand are coupled to a voltage measuring circuit, which may be disposed onthe vessel or a different vessel. The voltages imparted into thereceiver electrodes are then analyzed to infer the structure andelectrical properties of the Earth formations in the subsurface.

Another technique for electromagnetic surveying of subsurface Earthformations known in the art is transient controlled sourceelectromagnetic surveying (t-CSEM). In t-CSEM, electric current isimparted into the Earth at the Earth's surface (or sea floor), in amanner similar to f-CSEM. The electric current may be direct current(DC). At a selected time, the electric current is switched off, switchedon, or has its polarity changed, and induced voltages and/or magneticfields are measured, typically with respect to time over a selected timeinterval, at the Earth's surface or water surface. Alternative switchingstrategies are possible; as will be explained in more detail below.Structure of the subsurface is inferred by the time distribution of theinduced voltages and/or magnetic fields. t-CSEM techniques aredescribed, for example, in Strack, K.-M., 1992, Exploration with deeptransient electromagnetics, Elsevier, 373 pp. (reprinted 1999).

SUMMARY OF THE INVENTION

A marine electromagnetic sensor cable according to one aspect of theinvention includes a plurality of sensor modules disposed at spacedapart locations along a cable. Each module includes at least onemagnetic field sensor and at least one pair of electrodes associatedwith the module. The electrodes are arranged to measure electric fieldin a direction along the direction of the cable. The cable is arrangedto form a closed pattern.

A marine electromagnetic surveying system according to another aspect ofthe invention includes a vessel towing at least one electromagneticantenna in a body of water. A controllable source of electric current isdisposed on the vessel. The system includes a plurality of sensormodules disposed at spaced apart locations along a cable. Each moduleincludes at least one magnetic field sensor and at least one pair ofelectrodes associated therewith. The electrodes are arranged to measureelectric field in a direction along the direction of the cable. Thecable is arranged to form a closed pattern. The system includes meansfor recording signals generated by the electrodes and magnetic fieldsensors in response to electromagnetic energy imparted into the Earth'ssubsurface by passing electric current through the at least one antenna.

Another aspect of the invention is a marine electromagnetic sensor cableincluding a plurality of sensor modules disposed at spaced apartlocations along a cable. Each module includes at least one pair ofelectrodes associated therewith. The electrodes are arranged to measureelectric field in a direction along the direction of the cable. Aplurality of spaced apart magnetic field sensors is associated with eachmodule and arranged to enable determining an electric field amplitude ina direction transverse to the direction of the cable from magnetic fieldgradient.

A marine electromagnetic surveying system according to another aspect ofthe invention includes a vessel towing at least one electromagneticantenna in a body of water. A controllable source of electric current isdisposed on the vessel. The system includes a plurality of sensormodules disposed at spaced apart locations along a cable. The sensorcable including a plurality of sensor modules disposed at spaced apartlocations along a cable. Each module includes at least one pair ofelectrodes associated therewith. The electrodes are arranged to measureelectric field in a direction along the direction of the cable. Aplurality of spaced apart magnetic field sensors is associated with eachmodule and arranged to enable determining an electric field amplitude ina direction transverse to the direction of the cable from magnetic fieldgradient.

A method for measuring magnetotelluric response of the Earth'ssubsurface according to another aspect of the invention includesmeasuring transient controlled source electromagnetic response of theEarth's subsurface below the bottom of a body of water over a pluralityof actuations of an electromagnetic transmitter. The transient responsemeasurements are stacked and the stacked transient response issubtracted from measurements of total electromagnetic Earth responseover a time period including the plurality of transient responsemeasurements to generate the magnetotelluric response.

A method for determining a component of electric field response to atime varying electromagnetic field induced in the Earth's subsurfaceaccording to another aspect of the invention includes measuring magneticfield gradient in at least two orthogonal directions in response to theinduced electromagnetic field and determining an electric field responsein a direction normal to the magnetic field gradient measurements.

Another aspect of the invention is a method for determining a componentof electric field response of the Earth's subsurface to a time varyingelectromagnetic field induced in the Earth's subsurface. A methodaccording to this aspect of the invention includes measuring electricfield response along a substantially closed pattern on at least one ofthe Earth's surface and the bottom of a body of water and determining anelectric field response in a direction normal to the measured electricfield response using electric field response measurements made atopposed positions along the closed pattern.

Other aspects and advantages of the invention will be apparent from thefollowing description and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a marine EM acquisition system that may include acquisitionmodules according to various aspects of the invention.

FIG. 2 shows one example of an acquisition module that may be used withthe system shown in FIG. 1.

FIG. 3 shows another example of an acquisition module.

FIG. 4 shows another example of an acquisition system.

FIG. 5 shows inducing magnetic fields in the Earth's subsurface.

DETAILED DESCRIPTION

One example of a marine electromagnetic (EM) survey acquisition systemis shown schematically in FIG. 1. The system may include a survey vessel10 that moves along the surface of a body of water 11 such as a lake orthe ocean. The survey vessel 10 includes thereon certain equipment,shown generally at 12 and referred to for convenience as a “recordingsystem.” The recording system 12 may include (none of the followingshown separately for clarity of the illustration) navigation devices,source actuation and control equipment, and devices for recording andprocessing measurements made by various sensors in the acquisitionsystem. The vessel 10 may tow a seismic energy source 14 such as an airgun or an array of such air guns, a vertical electric dipole “source”antenna 19 including vertically spaced apart electrodes 16C, 16D and ahorizontal electric dipole “source” antenna 17, which may includehorizontally spaced apart electrodes 16A, 16B. The vertical electrodes16C, 16D are typically energized by current flowing through one of thelines going from either electrode 16C or 16D to the survey vessel 10.The other line may be electrically inactive and only used to keep thevertical dipole antenna in is preferred shape. The electrodes on thesource antennas 17, 19 may be referred to as “source electrodes” forconvenience. The recording system 12 may include a controllable powersupply (not shown separately) to energize the source electrodes for thepurpose of inducing electromagnetic fields in the subsurface below thewater bottom 13.

In the present example the source electrodes 16A, 16B and 16C, 16D,respectively on each antenna 17, 19, can be spaced apart about 50meters, and can be energized by the power supply (not shown) such thatabout 1000 Amperes of current flows through the electrodes. This is anequivalent source moment to that generated in typical electromagneticsurvey practice known in the art using a 100 meter long transmitterdipole, and using 500 Amperes current. In either case the source momentcan be about 5×10⁴ Ampere-meters. The source moment used in anyparticular implementation is not intended to limit the scope of thisinvention.

If the system is configured to record transient EM signals, the electriccurrent used to energize the source electrodes can be direct current(DC) switched off at a particular time or at particular times. Suchswitching time may be conveniently correlated to a signal recording timeindex equal to zero. It should be understood, however, that switching DCoff is only one implementation of electric current switching that isoperable to induce transient electromagnetic effects in the Earth'ssubsurface. In other examples, the electric current (DC) may be switchedon, may be switched from one polarity to the other (bipolar switching),or may be switched in a pseudo-random binary sequence (PRBS) or anyhybrid derivative of such switching sequences. See, for example, Duncan,P. M., Hwang, A., Edwards, R. N., Bailey, R. C., and Garland, G. D.,1980, The development and applications of a wide band electromagneticsounding system using pseudo-noise source, Geophysics, 45, 1276-1296 fora description of PBRS switching. The system may also be configured torecord “frequency domain” signals in conjunction with or alternativelyto recording transient signals. The power supply (not shown) may in suchinstances generate a continuous alternating current having one or moreselected component frequencies to perform such frequency domainelectromagnetic surveying.

The recording system 12 may include equipment (the source controller)that may actuate the seismic source 14 at selected times and may includedevices that record, or accept recordings for processing, from seismicsensors (explained below with reference to FIG. 2) that may be disposedin a sensor cable 24 or elsewhere in the acquisition system.

In the present example, the sensor cable 24 is shown disposed on thewater bottom 13 for making measurements corresponding to Earthformations below the water bottom 13. The sensor cable 24 may includethereon a plurality of longitudinally spaced apart sensor modules 22.Examples of components in each sensor module 22 will be furtherexplained below with reference to FIGS. 2 and 3. Each sensor module 22may have inserted into an upper side thereof a substantially verticallyextending sensor arm 22A. Details of one example of the verticallyextending sensor arm 22A will be explained below with reference to FIG.3. Preferably the vertically extending sensor arm 22A includes thereinor thereon some type of buoyancy device or structure (not shownseparately) to assist in keeping the sensor arm 22A in a substantiallyvertical orientation with respect to gravity. Each sensor module 22 mayinclude extending from its lower side a spike 22C as described, forexample, in Scholl, C. and Edwards, N., 2007, Marine downhole toseafloor dipole-dipole electromagnetic methods and the resolution ofresistive targets, Geophysics, 72, WA39, for penetrating the sedimentsthat exist on the water bottom 13 to a selected depth therein. Disposedabout the exterior of portions of the sensor cable 24 adjacent eachlongitudinal end of each sensor module 22 may be galvanic electrodes 23which are used to measure voltages related to certain components ofelectric field response to induced electromagnetic fields in thesubsurface. In the present example, laterally extending sensing arms 22Bmay be disposed from one or both the sides of each sensor module 22.Such sensing arms 22B will be explained in more detail with reference toFIG. 3. The sensor cable 24 may in some implementations be disposed onthe water bottom 13 in a closed pattern that will be further explainedwith reference to FIG. 4.

Signals acquired by various sensing devices associated with each module22 and the cable 24 may be transmitted to and stored in a recoding node26. Such transmission may be made by including in the cable 24 one ormore electrical and/or optical conductors (not shown) to carryelectrical power and/or data signals. The recording node 26 may bedisposed on the water bottom 13 as shown on disposed in a buoy (notshown) at the discretion of the system designer. The recording node 26may include any form of data storage device, for example aterabyte-sized hard drive or solid state memory. If disposed on thewater bottom 13 as shown in FIG. 1, the recording node 26 may beretrieved from the water bottom 13 by the vessel 10 to interrogate thestorage device (not shown), or the storage device (not shown) may beaccessed for interrogation by connecting a data transfer cable (notshown) to a suitable connector or port (not shown) on the recording node26. The manner of data storage and transfer with respect to the node 26may be according to well known art and are not intended to limit thescope of this invention.

One example of the sensor module 22 is shown in cut away view in FIG. 2.The sensor module 22 may include a sealed, pressure resistant housing 28affixed to the cable 24 at a selected position along the cable 24. Thehousing 28 may be affixed to the cable 24 by splicing within the cable,by molding the housing 28 thereon or by using water tight, pressureresistant electrical and mechanical connectors on each of the cable 24and housing 28, such as a connector shown in U.S. Pat. No. 7,113,448issued to Scott.

The interior of the housing 28 may define a pressure sealed compartmentthat may include some or all of the components described below. Sensingelements in the module 22 may include a three-axis magnetometer M thatincludes horizontal Mx, My and vertical Mz component magnetic fieldsensors. A three component seismic particle motion sensor G may also bedisposed in the housing 28. The seismic particle motion sensor G mayinclude three mutually orthogonal motion sensors Gx, Gy, Gz such asgeophones or accelerometers. The seismic sensor G detects particlemotion components of a seismic wavefield induced by the seismic source(14 in FIG. 1). The sensor module 22 may also include a hydrophone 30 inpressure communication with the water (11 in FIG. 1) for detecting thepressure component of the seismic wavefield induced by the seismicsource (14 in FIG. 1). The sensor module 22 may also include a gravitysensor GR within the housing 28. The sensor module 22 may includevoltage measuring circuits 39, 40 to measure voltages impressed acrosspairs of galvanic electrodes (23 in FIG. 1) disposed on opposed sides ofthe module 22 along the cable 24. In the present example, the electrodepairs may also include an electrode disposed along or at the end of eachof the vertical sensing arm 22A (the electrode shown at 23B) and thespike 22C (the electrode shown at 23A). The vertical sensing arm 22A maybe coupled to the housing 28 in a manner as will be explained below withreference to FIG. 3.

Signals generated by each of the sensing devices described above mayenter a multiplexer 32. Output of the multiplexer 32 may be conductedthrough a preamplifier 34. The preamplifier may be coupled to the inputof an analog to digital converter (ADC) 36, which converts the analogvoltages from the preamplifier 34 into digital words for storing andprocessing by a central processor 38, which may be any microprocessorbased controller and associated data buffering and/or storage deviceknown in the art. Data represented by digital words may be formatted forsignal telemetry along the cable 24 to the recording node (26 in FIG. 1)for later retrieval and processing, such as by or in the recordingsystem (12 in FIG. 1). The sensor module 22 may also include one or morehigh frequency magnetometers MH in signal communication with themultiplexer 32 and the components coupled to the output thereof.

The example sensor module 22 of FIG. 2 is shown in plan view in FIG. 3.The horizontal sensing arms 42 (also shown as 22B in FIG. 1) may becoupled to the housing 28 using pressure-sealed electrical connectors42A that mate with corresponding connectors 41 in the housing 28. Thesensing arms 42 may alternatively be permanently attached to the sensormodule 22 and foldable as well. The connectors 42A, 41 include one ormore insulated electrical contacts to communicate power and/or signalsto various sensing elements in the horizontal sensor arms 42. Thesensing elements may include a plurality of spaced apart single ormulti-axis magnetic field sensors 44, and a galvanic electrode 46. Thevertical sensing arm 22A may be similarly configured to have anelectrode and multiple magnetic field sensors. The spike (22C in FIG. 2)may be similarly instrumented with such sensing devices. The varioussensor arms and the spike may be configured such that they may belockingly and quickly installed into the housing as shown as the cable24 is extended into the water (11 in FIG. 1) from the survey vessel (10in FIG. 1).

Configured as explained with reference to FIGS. 2 and 3, the sensormodule 22 includes sensing devices to measure electric field in threedimensions, magnetic field in three dimensions and magnetic fieldgradient in at least two directions. Magnetic field gradient may bemeasured along the direction of the cable 24 (the third direction) bymeasuring difference between magnetic field measurements made inadjacent modules 22, or between successively more spaced apart modules22 along the cable 24. By measuring spatial components of magnetic fieldgradient, it may be possible to determine components of electric fieldin a direction transverse to the magnetic field gradient measurements.Ampere's law states that the spatial gradient of the magnetic field isequivalent to the derivative in time of the dielectric displacementfield plus the free current density, as shown in equation (1) below:

$\begin{matrix}{{\nabla{\times \overset{->}{H}}} = {\overset{->}{J} + \frac{\partial\overset{->}{D}}{\partial t}}} & (1)\end{matrix}$

Because the dielectric displacement field is coupled by the electricalpermittivity to the electric field E, the change with respect to time ofthe y-component of the electric field, Ey, field can be calculated ifthe spatial changes of the z-component of the magnetic field, Hz, withrespect to position along the cable, x, and cable direction spatialchange in magnetic field, Hx, with respect to vertical, z, are known.Thus, by measuring magnetic field gradient along selected directionsusing a cable system as shown herein, it is possible to determine atransverse component of the electric field.

One example of deployment of a cable system is shown in FIG. 4. Thecable 24 may include a tail buoy 48 at its distal end from the recordingnode 26, and may be disposed on the water bottom in a substantiallyclosed pattern. Note that the system shown in FIG. 4 may omit thehorizontal sensing arms 42 for determining transverse components of theelectric field. This is because the sum of the electric field componentswithin a closed loop is equal to zero. As a result, when the electrodes23 are disposed in a closed pattern as shown in FIG. 4, transversecomponents of electric field between laterally opposed pairs ofelectrodes (positions along the closed pattern) may be inferred from theelectric field measurements made between the longitudinally spacedelectrodes at such opposed positions. Alternatively, the horizontalsensing arms 42 may be included and measurements of electric field andmagnetic field gradient may be used to quality check the determinationof lateral electric field component determined by magnetic fieldgradient measurement and by electric field difference determination atselected positions along the closed pattern.

If components of the electric field transverse to the direction of thecable are determined by measuring magnetic field gradient or by usingtransversely mounted sensing arms, it may be possible to conduct asurvey without having the cable in a closed loop configuration as shownin FIG. 4. The system shown in FIG. 4 may provide certain advantages asexplained above.

The system shown in FIG. 1 includes horizontal and/or vertical electricdipole antennas for inducing an electric field in the Earth'ssubsurface, wherein electric and magnetic responses of the Earth aremeasured. It should be understood that the invention is equallyapplicable where magnetic fields are induced. Referring to FIG. 5, thesurvey vessel 10 may tow loop antennas 21A 21B at the end of a cable 21.The recording system 12 may pass electrical current through horizontalloop antenna 21A to induce a vertical magnetic field m_(A) in thesubsurface, and through vertical loop antenna 21B to induce a horizontalmagnetic field m_(B) in the subsurface. Measurements made by the varioussensing devices in the system (see FIGS. 2 through 4) may be made inresponse to such magnetic fields. Magnetic fields may be induced inaddition to as well as an alternative to electric fields for anyparticular electromagnetic survey.

It will also be apparent to those skilled in the art that the sensorcable (24 in FIG. 1) can also be arranged in a line, particularly wherethe horizontal sensing arms are used, and/or where spaced apart magneticfield sensors are used to determine transverse components of electricfield from the magnetic field gradient.

The sensor cable 24 may also be used with magnetotelluric measurementmethods and is not limited to controlled source electromagneticmeasurement methods. In one example of a method according to theinvention, a plurality of transient controlled source electromagneticmeasurements (t-CSEM), including one or more of electric field andmagnetic field are made along one or more selected directions using acable as shown in FIG. 4. Preferably, such measurements of electric andmagnetic field are made along three orthogonal directions. For suchplurality of measurements, preferably the source antenna (FIG. 1) is ina substantially fixed position. The electric and magnetic fieldmeasurements are summed or stacked. The result of the stacking is a highquality t-CSEM signal response. The stacked t-CESM signal response maythen be subtracted from the signals recorded over a substantial periodof time. The result will be the magnetotelluric (MT) response measuredby all the various sensing elements on the cable. The MT response may beprocessed according to techniques known in the art. See, for example,U.S. Pat. No. 6,950,747 issued to Byerly.

When MT response is determined as explained above, and processedaccording to one or more techniques known in the art, it then becomespossible to perform a joint inversion of the t-CSEM and MT responses. Iffrequency domain electromagnetic response is measured, such response mayalso be jointly inverted. Joint inversion is described, for example, inU.S. Pat. No. 5,870,690 issued to Frenkel et al. A particular benefitthat may be provided by making both CSEM and MT measurements from thesame sets of sensing devices, and processed through the same electroniccircuitry, for the purposes of join inversion is that the degree ofscaling or other response matching that would be required if the MT andCSEM responses were measured using separate systems, is substantiallyreduced.

Using a sensor cable as shown herein, it is also possible to performelectric field mapping in order to correct the MT response measurementsfor static shifts. See, for example, Stemberg, B. K., Washburne, J. C.and Pellerin, L., 1988, Correction for the static shift inmagnetotellurics using transient electromagnetic soundings, Geophysics,Volume 53, Issue 11, pp. 1459-1468. Prior to having a cable as explainedherein, the technique disclosed in the foregoing publication was onlyapplicable for land-based surveys. Using a cable and method according tothe invention, however, it is possible to correct the MT response forstatics using the t-CSEM response measured by the same sensing elementsin the sensor cable disposed on the sea floor. See also, Torres-Verdin,C, 1991, Continuous profiling of magnetotelluric fields, Ph.D. Thesis,University of California, and Torres-Verdin, C. and Bostick Jr, F. X.,1992, Principles of spatial surface electric field filtering inmagnetotellurics: Electromagnetic array profiling (EMAP), Geophysics,Volume 57, Issue 4, pp. 603-622. As explained in one or more of theforegoing publications, the MT response may be subject to verticalshifting in the log domain. Such shifting is caused by relativelyconductive or resistive “patches” of formation close to the waterbottom. The t-CSEM response is substantially unaffected by such patches,however, and may be used to calibrate the MT response for the effects ofsuch patches.

A sensor cable and EM measurement system and methods according to thevarious aspects of the invention may provide more electromagneticmeasurement components with data quality checking capabilities, and maybe easier to deploy than other EM cable systems and separateEM/seismic/gravity/magnetic cable sensing systems.

While the invention has been described with respect to a limited numberof embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed herein.Accordingly, the scope of the invention should be limited only by theattached claims.

1. A method for measuring magnetotelluric response of the Earth'ssubsurface, comprising: measuring transient controlled sourceelectromagnetic response of the Earth's subsurface below the bottom of abody of water over a plurality of actuations of an electromagnetictransmitter; stacking the transient response measurements; andsubtracting the stacked transient response from measurements of totalelectromagnetic Earth response over a time period including theplurality of transient response measurements to generate themagnetotelluric response.
 2. The method of claim 1 wherein the transientcontrolled source electromagnetic response is produced by inducing atransient magnetic field in the Earth's subsurface.
 3. The method ofclaim 1 wherein the transient controlled source electromagnetic responseis produced by inducing a transient electric field in the Earth'ssubsurface.
 4. The method of claim 1 further comprising staticcorrecting the magnetotelluric response using the at least one of thetransient response measurements and the stacked transient responsemeasurements.
 5. The method of claim 1 further comprising jointlyinverting the magnetotelluric response and the transient responsemeasurements.